In the baseline profile of H.264/AVC, two picture coding techniques are available. The first, intra coding, predicts the content of the input video picture (i.e., the picture being coded) using decoded luma and chroma samples from regions in the picture that have been coded earlier (and would therefore be available to a decoder). The second, predictive coding, predicts the content of the input video picture using previously coded pictures called reference pictures.
A traditional videoconference may begin with an intra-coded or I picture followed by predicted or P pictures. Unlike applications requiring random access (e.g., digital video recordings) which insert periodic I pictures, a videoconference typically uses one I picture at the beginning of transmission and never again (IPPP coding), except in special situations (e.g., error recovery). Hierarchical-P picture (HPP) coding includes a prediction structure that does not always reference the picture immediately preceding the input. Rather, the prediction structure in HPP coding is constructed so that the input pictures are decomposed into sets or temporal layers that offer temporal scalability.